Bangkok — Up to 75 percent of the security cameras were broken along the getaway path taken by the main suspect in last week's deadly Bangkok bombing, Thailand's police chief said Monday, revealing a major obstacle to an investigation that has only fuzzy images of the still-unidentified man.

Investigators are trying to "put pieces of the puzzle together" but have had to use their imagination to fill holes left by cameras that failed to record his movements, said national police chief Somyot Poompanmoung, openly frustrated as he spoke to reporters one week after the blast.

"For example, the perpetrator was driving away — escaping — and there are CCTV cameras following him. Sometimes there were 20 cameras on the street but only five worked," Somyot said. "Fifteen were broken, for whatever reason, they didn't work."

Another challenge is that investigators lack sophisticated equipment like police use on the popular TV crime series "CSI" to render blurry video clear, he said.

"Have you seen 'CSI'?" Somyot asked reporters. "We don't have those things."

One week after last Monday's bombing at the capital's revered Erawan Shrine, which left 20 people dead and more than 120 injured, police appeared no closer to tracking down suspects or determining a motive for the attack.

At 6:55 p.m. Monday, a crowd gathered at the shrine in central Bangkok and observed a minute of silence to mark the moment the bomb exploded. Chanting Buddhist monks led prayers as onlookers held lit candles to commemorate what authorities have called the deadliest attack in modern Thai history.

Police have faced criticism for sending mixed messages and stating theories as fact, only to later retract them, adding to the confusion at a time of public concern.

Grilled about what progress has been made in a week, the police chief said that basic questions about the suspect's identity and whereabouts remain unknown.

Asked if he is still in Thailand, Somyot said, "I don't know."

"I still believe he is in Thailand because I have no evidence to confirm otherwise," he said. Over the weekend, police spokesman Prawut Thawornsiri said he suspected the man may have left the country.

Police have released an artist's sketch of the suspect who was seen in security camera video from the open-air shrine leaving a backpack at a bench and walking away 15 minutes before the explosion. A separate camera showed the suspect, wearing a yellow T-shirt, on the back of a motorcycle taxi leaving the site.

"We only have those pictures," said Prawuth, the police spokesman. "The problem is the pictures aren't clear."

Police have questioned a motorcycle taxi driver believed to have driven the suspect away. The driver told police the man handed him a piece of paper saying "Lumpini Park," the city's largest park, which is near the shrine.

But none of the security cameras worked along the route to the park, Somyot said.

Police have tried to chase a report that after the suspect got off the motorcycle at the park, he went into a nearby hospital and changed into a grey T-shirt, Somyot said.

"We went to see if this is true and checked the CCTV cameras at Chulalungkorn Hospital. But we found that all the cameras were broken," he said, laughing nervously, and asking for understanding because police had been working nonstop and were tired.

"Since the bombing, we've barely slept," he said. "We're doing the best we can."

On Friday, Prime Minister Prayuth Chan-ocha said he had received offers of assistance from the US Embassy in Bangkok to help track down suspects and had assigned his deputy "to cooperate on borrowing equipment that includes facial-recognition technology."

However, Prayuth ruled out working with US investigators, insisting Thais can do the job.

The attack has raised concerns about safety in the capital, which attracts millions of tourists, and has left the city on edge. Police have responded to several calls in the past week about unattended bags which turned out to be false alarms, and have tried to reassure the public and international community that Bangkok is safe.

On Monday, police said a worker digging at a construction site found a grenade buried in the ground and a police explosives squad was sent to defuse it. The grenade was found in a residential area far from the city center.

Police Lt. Sakon Rungkiatpaisarn said the grenade appeared to have been buried for a while and authorities "do not think it has anything to do with (last Monday's) bombing."